From: Bernard Brady <[email-masked]>
Date: Wed, 25 Aug 93 10:56:32 EDT

Alan,

In trying to understand the SSP messages associated with the flow
diagrams you presented I am confused on the actual Data Link IDs that
is used in establishing a circuit. 

Case 1:

 LAN                          DLSw                          LAN
                              Origin/Target                              
         
 TEST CMD             CANUREACH          TEST CMD
  (D=0,S=4)             (Data Link ID)             (D=0,S=4)
  ------------>  		 -----------------> 	      ----------->

TEST RSP            ICANUREACH  	     TEST RSP
(D=4,S=0)           (Data Link ID,                 (D=4,S=0)
			      Target Circuit ID)    	 

<---------------       <--------------------  	    <---------------
                                                 
		             REACH_ACK
			     (Data Link ID,
                              Origin Circuit ID,
                             Target Circuit ID)    
			     -------------->	
 Circuit Est.
				
XID CMD               XID  CMD                     XID CMD
(D=4,S=4)           (Data Link ID,                   (D=4,S=4)
                              Origin Circuit ID,
                              Target Circuit ID)   
 ------------>  		 -----------------> 	    -----------> 

XID RSP               XID  RSP                     XID RSP
 (D=4,S=4)           (Data Link ID,                   (D=4,S=4)
                              Origin Circuit ID,
                              Target Circuit ID)   

<---------------       <--------------------  	    <---------------

Since the Data LINK ID contains the Origin and Target
Link SAP I take it that he Data Link ID contains DSAP=4 over the
established circuit above based on the value of S.  By the RFC the DSAP
is set equal to the SSAPand their isn't an issue.

Case 2:

 LAN                          DLSw                          LAN
                              Origin/Target                              
         
 TEST CMD             CANUREACH          TEST CMD
  (D=0,S=4)             (Data Link ID)             (D=0,S=4)
  ------------>  		 -----------------> 	     ----------->

 TEST CMD             CANUREACH          TEST CMD
  (D=0,S=8)             (Data Link ID)             (D=0,S=8)
  ------------>  		 -----------------> 	     ----------->

TEST RSP            ICANUREACH  	     TEST RSP
 (D=0,S=8)               (Data Link ID,                 (D=0,S=8)
			      Target Circuit ID)    	 
<---------------       <--------------------  	    <---------------

 				REACH_ACK
			     (Data Link ID,
                              Origin Circuit ID,
                             Target Circuit ID)    
			     -------------->	

TEST RSP            ICANUREACH  	     TEST RSP
 (D=8,S=0)               (Data Link ID,                 (D=8,S=0)
			      Target Circuit ID)    	 
<---------------       <--------------------  	    <---------------
                                                 
		             REACH_ACK
			     (Data Link ID,
                              Origin Circuit ID,
                             Target Circuit ID)    
			     -------------->	
Two Circuits Est.

XID CMD               XID  CMD                     XID CMD
 (D=4,S=4)           (Data Link ID,                   (D=4,S=4)
                              Origin Circuit ID,
                              Target Circuit ID)   
 ------------>  		 -----------------> 	    -----------> 

XID RSP               XID  RSP                     XID RSP
 (D=4,S=4)           (Data Link ID,                   (D=4,S=4)
                              Origin Circuit ID,
                              Target Circuit ID)   
<---------------       <--------------------  	    <---------------

XID CMD               XID  CMD                     XID CMD
 (D=4,S=8)           (Data Link ID,                   (D=4,S=8)
                              Origin Circuit ID,
                              Target Circuit ID)   
 ------------>  		 -----------------> 	    -----------> 

XID RSP               XID  RSP                     XID RSP
 (D=4,S=8)           (Data Link ID,                   (D=8,S=4)
                              Origin Circuit ID,
                              Target Circuit ID)    
<---------------       <--------------------  	    <---------------

 By the RFC the DSAP is set equal to the SSAP.  In this case there is an
issue in the what is contained in the Data Link ID in the DSAP field 4
or 8?  It make more sense that it should always be set to 4 if the SSAP
is a multiple of 4.  Is this correct interpretation?

Bernie